test event handler of fish shell script

test_fish_event_handler.fish

function custom_event_test --on-event test_event
    echo 'event hook'
    echo event test: $argv
end

emit test_event foo bar


function on_pwd_changed --on-variable PWD
    echo 'on_pwd_changed hook'
    echo ">>> The current dir is: " $PWD
end

cd ../
cd -

function on_foo_changed --on-variable foo
    echo ">>> foo is: " $foo
end

set foo bar
set foo foobarz


#### run it: `fish test_fish_event_handler.fish`

https://fishshell.com/docs/current/language.html#event-handlers
https://brettterpstra.com/2019/10/15/fish-shell-fun-event-handlers/

also see https://github.com/jethrokuan/z/blob/master/conf.d/z.fish#L45

function __z_on_variable_pwd --on-variable PWD
    __z_add
end